Display devices using liquid crystalline compounds have been extensively used in the display for watches, calculators, word processors and so forth. These display devices are those utilizing optical anisotropy, dielectric anisotropy, etc. of liquid crystalline compounds. Liquid crystal phases include a nematic phase, a smectic phase and a cholesteric phase, but display devices utilizing the nematic phase have been most extensively used. Display modes include a dynamic scattering (DS) mode, a deforming of aligned phases (DAP) mode, a guest-host (GH) mode, a twisted nematic (TN) mode, a supertwisted nematic (STN) mode, a thin film transistor (TFT) mode and the like.
Liquid crystalline compounds used for these display modes should exhibit liquid crystal phases within a broad temperature range around room temperature, have a sufficient stability under conditions where the display devices are used, and also have sufficient characteristics for driving the display devices, but at present, no single liquid crystalline compound satisfying these conditions has been found. Therefore, the fact is that several kinds to dozens of kinds of liquid crystalline compounds have been blended to prepare liquid crystal compositions furnished with the required characteristics. These liquid crystal compositions are required to be stable to moisture, light, heat and air which are usually present under conditions where the display devices are used, to be also stable to electric field or electromagnetic irradiation, and moreover, to be chemically stable to the blended compounds. It is further required for the liquid crystal compositions that values of various physical properties such as optical anisotropy value (.DELTA.n), dielectric anisotropy value (.DELTA..epsilon.), etc. are suitable, depending upon the display mode and the shape of the display devices. Furthermore, it is important for the respective components in the liquid crystal compositions to have good compatibility.
In recent years, a TFT type display having characteristics such as high contrast, broad view angle or the like has been studied extensively. Liquid crystal compositions for TFT have required such physical properties as high voltage holding ratio, small temperature dependency of the ratio, broad liquid crystal phase temperature range, low threshold voltage (Vth), small temperature dependency of the voltage, good compatibility with other liquid crystal materials or low viscosity. Alternatively, the compositions having high An are also useful to improve the response rate.
For this purpose, suitable is a polycyclic compound containing at its end rings substituted with a fluorine atom or a fluoro-substituted alkyl group, many syntheses and studies of which have been done extensively.
For instance, Japanese Patent Publication No. Sho 63-13411 discloses tricyclic compounds containing a fluorophenyl group at the terminal, Japanese Patent Publication No. Sho 63-44132 discloses tricyclic compounds containing a difluorophenyl group at the terminal, Japanese Patent Kokai No. Hei 2-233626 discloses di- to tetra-cyclic compounds containing a trifluorophenyl group, Japanese Patent Publication No. Hei 3-500413 discloses polycyclic compounds containing a difluoromethoxyphenyl group, and Japanese Patent Publication No. Hei 2-501311 discloses polycyclic compounds containing a trifluoromethoxyphenyl group. Each of the above publications also discloses liquid crystal compositions containing such compounds.
DE 4111991 discloses polycyclic compounds containing a fluoroethoxyphenyl group at the terminal, DE 4137401 and DE 4142519 disclose polycyclic compounds containing a fluoropropoxyphenyl group, the claims of which include a great number of compounds, but the compounds as concretely synthesized or illustrated are a very limited number of compounds comprising a combination of the specified ring and bridge.
An object of the present invention is to provide a liquid crystalline compound having exceedingly high voltage holding ratio, exceedingly small temperature dependency of the ratio, low threshold voltage, high An and excellent compatibility with other liquid crystal materials, especially at low temperatures, a liquid crystal composition containing said compound as a constituent and a liquid crystal display device composed of said liquid crystal composition.
As a result of our intensive studies to achieve the above object, it was found surprisingly that a group of compounds included within the claims of said DE patents, but not illustrated concretely therein satisfy various characteristics required for said liquid crystalline compounds and have excellent compatibility with other liquid crystalline compounds, especially at low temperatures, thus leading to completion of the present invention.